Steel wire rod for hard drawn spring,drawn wire rod for hard drawn spring and hard drawn spring, and method for producing hard drawn spring

ABSTRACT

Disclosed is a steel wire rod for hard-drawn springs capable of exhibiting fatigue strength and sag resistance equivalent to or higher than springs made of an oil-tempered wire. The steel wire rod contains carbon in a range from 0.5 to less than 0.7 mass %, silicon in a range from 1.4 to 2.5 mass %, manganese in a range from 0.5 to 1.5 mass %, chromium in a range from 0.05 to 2.0 mass %, and vanadium in a range from 0.05 to 0.40 mass %, and has an area ratio Rp with respect to pearlite which satisfies the mathematical expression (1):  
       Rp ( area %)≧55×[ C ]+61   (1)  
     where [C] denotes the content (mass %) of carbon.

TECHNICAL FIELD

[0001] This invention relates to a steel wire rod for hard-drawn springswhich is useful as a material for valve springs, clutch springs, brakesprings, etc. of automotive engines, a wire for hard-drawn springs usingsuch a wire rod, hard-drawn springs, and a useful method for producingsuch hard-drawn springs.

BACKGROUND ART

[0002] As development of light-weighted construction and highperformance for automotive vehicles, etc. has progressed, high stressdesign has been required for valve springs, clutch springs, brakesprings or the like. Springs excellent in fatigue strength and sagresistance have been demanded. In particular, there is a strong demandfor high stress design of valve springs.

[0003] Recently, it has been a custom that valve springs are primarilyproduced by cold coiling an oil-tempered wire that has been applied withquenching and tempering. According to the Japanese Industry Standards(JIS), for example, an oil-tempered wire (according to JIS G3561) forvalve springs is separately defined from an ordinary oil-tempered wire(according to JIS G3560). Thus, it is required to strictly control thekind of steel, allowable impurity content, depth of flaw, etc.

[0004] The oil-tempered wire has the following advantage anddisadvantage. As regards the advantage, since the oil-tempered wire hastempered martensite, it can provide springs of high strength, and it hasexcellent fatigue strength and sag resistance. As regards thedisadvantage, there is required a large-scaled facility and cost forheat treatment such as quenching and tempering to produce theoil-tempered wire.

[0005] Some of valve springs of low load stress are obtained by drawingcarbon steel that has ferrite/pearlite or pearlite to increase strength(also called “hard-drawn wire”), and by cold coiling the hard-drawnwire. According to the JIS, such a wire belongs to the criteria of“Piano Wire Type V” for “valve springs or like springs” in the criteriaof piano wires according to JIS G3522.

[0006] Springs made of the aforementioned hard-drawn wire (hereinafter,referred to as “hard-drawn springs”) can be produced with a low costbecause heat treatment is not required in the production method.However, the wire in which ferrite/pearlite or pearlite has beensubjected to drawing has low fatigue properties and low sag resistance.Accordingly, even if such a wire is used for springs, it cannot providefor high-strength springs that are required in the recent technology.

[0007] There also have been studied various techniques to producehigh-strength hard-drawn springs in light of the advantage of low-costproduction. Japanese Unexamined Patent Publication No. HEI 11-199981proposes an exemplified method for obtaining cementite of a specificconfiguration by performing a wire drawing process to pearlite ineutectoid-hypereutectoid steel, which is usable as a piano wire havingproperties equivalent to an austempered wire. This method, however,unavoidably raises the production cost because the production process iscomplicated such that a step of changing the wire drawing direction isadditionally required.

[0008] In view of the above, an object of this invention is to provide asteel wire rod used for producing hard-drawn springs capable ofexhibiting fatigue strength and sag resistance equivalent to or higherthan springs produced by an oil-tempered wire, a wire for hard-drawnsprings, such hard-drawn springs, and a useful method for producing suchhard-drawn springs with a low cost.

DISCLOSURE OF THE INVENTION

[0009] An inventive steel wire rod for hard-drawn springs that hasaccomplished the above object contains carbon in a range from 0.5 toless than 0.7 mass %, silicon in a range from 1.4 to 2.5 mass %,manganese in a range from 0.5 to 1.5 mass %, chromium in a range from0.05 to 2.0 mass %, and vanadium in a range from 0.05 to 0.40 mass %,and has an area ratio Rp with respect to pearlite which satisfies themathematical expression (1):

Rp(area%)≧55×[C]+61  (1)

[0010] where [C] denotes the content (mass %) of carbon.

[0011] It is effective for the inventive steel wire rod (a) to containnickel in a range from 0.05 to 0.5 mass % or (b) to satisfy therequirement that the number of carbide and carbo-nitride of vanadium andchromium, complex carbide and complex carbo-nitride of vanadium andchromium each having a diameter of 50 nm or less in terms of ahypothetical circle, is not smaller than ten per unit area of μm² inlamellar ferrite. With this arrangement, the properties of thehard-drawn springs can be further improved.

[0012] An inventive wire for hard-drawn springs that has accomplishedthe above object contains carbon in a range from 0.5 to less than 0.7mass %, silicon in a range from 1.4 to 2.5 mass %, manganese in a rangefrom 0.5 to 1.5 mass %, chromium in a range from 0.05 to 2.0 mass %, andvanadium in a range from 0.05 to 0.40 mass %, and has an area ratio Rpwith respect to pearlite which satisfies the mathematical expression(1), and has a tensile strength TS which satisfies the mathematicalexpression (2):

Rp(area%)≧55×[C]+61   (1)

[0013] where [C] denotes the content (mass %) of carbon,

−13.1d ³+160d ²−671d+3200≧TS≧−13.1d ³+160d ²−671d+2800   (2)

[0014] where d is a diameter (mm) of the wire which satisfies theexpression [1.0≦d≦10.0].

[0015] It is effective for the wire to contain nickel in a range from0.05 to 0.5 mass %.

[0016] High-strength hard-drawn springs are producible by using theabove steel wire rod or the above wire. Further, preferably, thehard-drawn springs satisfy the requirement that a residual stress of thespring is changed from a compression to a tension at a depth of 0.05 mmor more from the inner surface of the spring. More preferably, thedepth-wise position of the spring is 0.15 mm or more from the innersurface of the spring. Furthermore, it is effective to apply a nitridingprocess on the hard-drawn springs.

[0017] In producing the aforementioned hard-drawn springs, it isdesirable to apply a stress τ (MPa) to the springs at a temperature notlower than a room temperature at least once after shot-peening, whereinthe stress τ satisfies the mathematical expression (3):

τ≧TS(MPa)×0.5   (3)

[0018] where TS denotes a tensile strength of the wire. In thisproduction method, preferably, the temperature at which the stress τ isapplied is 120° C. or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a graph showing a relationship between carbon contentand area ratio of pearlite in comparison of the inventive steel wire rodwith ordinary carbon steel wire rod.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] The inventors of this application have made extensive study andresearches in order to find out a steel material for hard-drawn springsthat can accomplish the above object of this invention from a variety ofangles. As a result of their study, they have found that a steel wirerod whose chemical composition has been strictly defined and whichsatisfies the range [range as defined in the aforementioned mathematicalexpression (1)] in terms of a relationship between the area ratio ofpearlite and the carbon content in the steel wire rod can producehard-drawn springs which exhibit fatigue strength and sag resistanceequivalent to or higher than springs using an oil-tempered wire, andaccomplished this invention.

[0021] The chemical composition of the inventive steel wire rod isrequired to be properly regulated in the following manner. The reasonsfor setting the range of each component are as follows.

[0022] C: 0.5 to Less Than 0.7 Mass %

[0023] Carbon is a useful element to enhance tensile strength of a wireand to secure certain fatigue properties and sag resistance of springs.An ordinary piano wire contains carbon of about 0.8 mass %. However, inproducing a wire of high strength, which is an object of this invention,if the content of carbon is 0.7 mass % or larger, a wire containingcarbon in such a high content has increased defect sensitivity, and acrack occurs from surface flaws and inclusion. As a result, fatigue lifeof springs made of such a wire may be shortened. In view of this, inthis invention, the content of carbon is set to be less than 0.7 mass %.On the other hand, if the content of carbon is less than 0.5 mass %, awire containing carbon in such a low content cannot provide a tensilestrength as required for producing high-strength springs. To makematters worse, in case of such a low carbon content, the content ofpro-eutectoid ferrite, which promotes generation of cracks due tofatigue, may increase, thereby deteriorating fatigue properties ofsprings. In view of this, it is necessary to set the lower limit ofcarbon content at 0.5 mass %.

[0024] Si: 1.4 to 2.5 Mass %

[0025] Silicon is an element which enhances a tensile strength of a wireby a solid solution strengthening and which contributes to improvementin fatigue properties and sag resistance of springs. It is necessary toincrease the content of silicon by an amount corresponding to thelowered content of carbon. In view of this, the lower limit of siliconcontent is set at 1.4 mass %. On the other hand, if the silicon contentexceeds 2.5 mass %, deoxidation, flaws or the like may increase on thespring surface, which may lower the fatigue strength of springs.Preferably, the lower limit of the silicon content is set at about 1.7mass %, and the upper limit thereof is set at about 2.2 mass %.

[0026] Mn: 0.5 to 1.5 Mass %

[0027] Manganese is an element which makes pearlite in fine and orderlymanner and which contributes to improvement in fatigue properties ofsprings. In order to allow springs to exhibit such effects, it isrequired to set the content of manganese at 0.5 mass % or more. However,an excessive content of manganese may likely to generate bainite at thetime of hot rolling or patenting, which resultantly may deterioratefatigue properties of springs. In view of this, the manganese content isrequired to be set at 1.5 mass % or lower. Preferably, the lower limitof the manganese content is set at about 0.7 mass %, and the upper limitthereof is set at about 1.0 mass %.

[0028] Cr: 0.05 to 2.0 Mass %

[0029] Chromium is an element which is useful in narrowing pearlitelamellar spacing, increasing strength after hot rolling or heattreatment, and in improving sag resistance of springs. In order to allowsprings to exhibit the above effects, it is required to set the contentof chromium at 0.05 mass % or more. An excessive content of chromium,however, may undesirably extend the period for patenting, anddeteriorate toughness and ductility of springs. In view of this, it isrequired to set the chromium content at 2.0 mass % or less.

[0030] V: 0.05 to 0.40 Mass %

[0031] Vanadium is an element which is useful to fine the nodule size ofpearlite and in improving wire drawability, toughness and sag resistanceof springs, etc. In order to allow springs to exhibit the above effects,it is required to set the vanadium content at 0.05 mass % or more.Preferably, the vanadium content is set at 0.10 mass % or more.Excessive content of vanadium, namely, vanadium content of more than0.40 mass % may likely to produce bainite at the time of hot rolling orpatenting, which may resultantly shorten fatigue life of the springs.

[0032] The basic chemical composition of the inventive steel wire rod isas mentioned above. It is possible to add nickel in the range from 0.05to 0.5 mass % according to needs. The range of nickel content and thereason for setting the range thereof are as follows.

[0033] Ni: 0.05 to 0.5 Mass %

[0034] Nickel is an element which is effective in lowering defectsensitivity, increasing toughness of springs, suppressing breakagetrouble at the time of coiling, and in improving fatigue life ofsprings. In order to allow nickel to exhibit the above effects, it ispreferable to contain nickel of 0.05 mass % or more. Excessive contentof nickel, however, may likely to produce bainite at the time of hotrolling or patenting, which resultantly may produce disadvantages ratherthan the above advantages. In view of this, the nickel content ispreferably set at 0.5 mass % or lower. Preferably, the lower limit ofnickel content is set at 0.15 mass %, and the upper limit thereof is setat 0.30 mass %.

[0035] The essential component of the remainder constituting theinventive steel wire rod other than the aforementioned chemicalcomponents is iron. However, it should be appreciated that minorcomponents other than the above components may be contained as far asthey do not interfere the properties of the steel material for producingthe inventive springs. In other words, a steel wire rod containing suchminor components falls in the scope of this invention. Some of theexemplified minor components are molybdenum in the content of about 0.5mass % or less, which is added for the purpose of obtaining improvedeffect due to hardenability, aluminum in the content of about 0.05 mass% or less, which is added as a deoxidizer at the time of producingsteel, and impurities, particularly, unavoidable impurities such asphosphorous, sulfur, arsenic, antimony, tin, etc. (e.g., 0.02% or lesswith respect to phosphorous or sulfur, and 0.01% or less with respect toarsenic, antimony, and tin).

[0036] It is required to set the composition of the inventive steel wirerod in an appropriate range [range as defined in the aforementionedmathematical expression (1)] in light of a relationship between the arearatio of pearlite and the carbon content in a steel wire rod. The reasonfor setting the range is as follows.

[0037] The carbon content of the steel used in this invention isrequired to be set not lower than 0.5 mass % to less than 0.7 mass % asmentioned above, namely, lower than the content of eutectoid component.When a wire rod is produced by using a steel having such a compositionaccording to a conventional method, pro-eutectoid ferrite is likely togenerate on the wire rod, and fatigue failure may occur from suchpro-eutectoid ferrite, thus shortening fatigue life of resultantsprings. In order to eliminate such a drawback, it is required tosuppress generation of pro-eutectoid ferrite as much as possible whileincreasing the ratio of pearlite.

[0038]FIG. 1 is a graph showing a relationship between the carboncontent and the area ratio of pearlite in a steel material. Whereasgenerally available carbon steel has a relatively low pearlite arearatio, the inventive steel wire rod has a relatively high pearlite arearatio considering its relation to the carbon content in view of theabove point.

[0039] It is effective to maximize the cooling rate of the wire rod in atemperature zone at least covering a transformation point A_(e3)(uppermost temperature at which austenite and ferrite can coexist in anequilibrium state) and a transformation point A_(e1) (uppermosttemperature at which ferrite and cementite can coexist) at the time ofhot rolling or patenting in order to obtain such a structure thatsatisfies the aforementioned mathematical expression (1). Specifically,in case of hot rolling, it is effective to control the cooling rate at5° C./sec or higher, preferably at 10° C./sec or higher in the abovetemperature zone as a cooling condition on a conveyor. It should benoted, however, that excessively continuing the cooling may fail toobtain fine pearlite, with the result that a structure such as bainiteas a result of super cooling may be intruded, thereby lowering toughnessof the resultant springs. In view of the above, it is recommended tomonitor the cooling condition each time the wire rod is carried over theconveyor at certain positions in such a manner that the cooling isgradually carried out until the temperature of the wire rod falls belowabout 550° C.

[0040] In case of patenting, the cooling rate from the transformationpoint A_(e3) to the transformation point A_(e1) is relatively fast.However, it is preferable to select a medium having large heatconductivity for performing isothermal transformation of the wire rod.Specifically, it is preferable to use a lead bath or a salt bath ratherthan a fluid bath. It is preferable to provide a cooling step between astep of austenitizing in a heating furnace and a step of performingisothermal transformation in a furnace to thereby forcibly performcooling by the cooling step in order to expedite cooling operation. Itis also effective to maximize the wire speed in order to accelerate thecooling rate. It is also possible to measure the pearlite area ratio Rpwith respect to a wire or with respect to springs as a final productbecause the pearlite area ratio Rp does not greatly vary depending on astatus as to whether a wire drawing process or a spring forming processwhich follows the wire drawing process has been implemented or not.

[0041] It is desirable to reinforce a ferrite part of a steel wire rod,which is the weakest part of pearlite since such reinforcement canimprove sag resistance of resultant springs. To reinforce ferrite meansto precipitate micro precipitants in the ferrite to such an extent thatthe sum of the number of precipitants, namely, carbide and carbonitrideof vanadium and chromium, complex carbide and complex carbonitride ofvanadium and chromium (hereinafter, all these kinds of precipitants aresometimes referred to as “complex carbonitride, etc.”) in which eachprecipitant has a diameter of 50 nm or less in terms of a hypotheticalcircle, is ten or more per μm². The term “diameter in terms of ahypothetical circle” means a diameter of a hypothetical circle whosearea is equivalent to the actual area of a corresponding precipitant.

[0042] As far as the inventive steel wire rod satisfies the aboverequirements, it is allowable for the inventive steel wire rod topartially contain complex carbide, etc. having a diameter larger than 50nm in terms of a hypothetical circle. However, it is preferable thatalmost or all the complex carbide etc. has a diameter of 50 nm orsmaller. The lower limit of the size of such complex carbides is notspecifically limited. However, considering the fact that the maximalsize of a substance recognizable by, e.g., a currently availabletransmission microscope of magnification at 150,000 is about 10 nm, itis true to say that about 10 nm is substantially a lower limit of ameasurable precipitant according to a current state of art.

[0043] It is preferable (1) to cool the wire rod at a cooling rate of 2°C./sec or more in a temperature zone from 900 to 800° C., which is anaustenitizing temperature zone, (namely, not to precipitate in theaustenitizing temperature zone) after rolling and then cool the wire rodat a cooling rate from 0.5 to 1.0° C./sec in a temperature zone of 750to 400° C. or (2) to transform the wire rod at 640° C. after heating thewire rod to 900° C., and then cool the wire rod at a cooling rate of 0.5to 1.0° C./sec until the temperature of the wire rod reaches 400° C. inorder to satisfy the above requirements while securely dispersing alarge amount of micro precipitants in the ferrite.

[0044] Drawing the steel wire rod having been treated as mentioned aboveinto a wire and coiling the wire enables to produce springs capable ofexhibiting the desirable properties. It is effective to allow the wireobtained by drawing the steel wire rod (hard-drawn wire for springs) tosatisfy the above mathematical expression (2) to allow the resultantsprings to exhibit the above effects more efficiently.

[0045] In association with the expression (2), the tensile strength of awire is defined in accordance with the diameter of a wire in thecriteria of JIS G3522-SWP-V. Specifically, tensile strength TS definedin the aforementioned criteria is lower than that in the criteria ofSWP-B, etc. designed for springs of general use. The reason for settingthe tensile strength of the wire according to the above standard lowerthan that for springs of general use is conceived as follows.Excessively high tensile strength may likely to increase defectsensitivity of wire, lower toughness and ductility of springs, and causeundesirable breaking of wire while drawing, breakage during coiling,fatigue failure, brittle fracture of springs, etc.

[0046] In view of the above, this invention has succeeded in producingsprings from the wire having the tensile strength TS equal to or largerthan the value in the right object of the expression (2), as well as useof such springs by lowering defect sensitivity and increasing toughnessand ductility. However, setting the tensile strength TS of the wireexcessively high may cause an adverse effect due to increased defectsensitivity or lowered toughness and ductility. In view of such adrawback, according to this invention, the value in the left object ofthe expression (2) is set as the upper limit of the tensile strength.Although a wire which satisfies the requirements in the expression (2)may be obtainable with use of a conventional wire drawing facility.However, considering a recent demand that a wire of an extremely highstrength is subjected to plastic deformation, it is desirable tooptimally set the requirements so as not to cause breaking of wire. Inview of this, the following matters should be considered: (1) a metallicsoap is used as a lubricant after coating the wire rod with phosphate asa pre-drawing process; (2) the reduction of area of each die used forwire drawing is set in a range from 15 to 25% (the reduction of area ofa die used in a final stage of wire drawing can be set lower than theaforementioned range in order to regulate a residual stress), and (3) awire drawing rate should not be exceedingly raised in order to preventtemperature rise during the drawing, etc.

[0047] In the inventive springs, it is preferable that a residual stressis changed from a compression to a tension at a depth of 0.05 mm or morefrom the inner surface of the spring, and preferably at a depth of 0.15mm or more. Generally, valve springs and high-strength springsequivalent thereto are used in a state where a compressive residualstress is exerted onto the spring surface by shot-peening. When such aresidual stress is measured stepwise in a depth direction of the springfrom the inner surface thereof, the measured residual stress turns froma compression to a tension at a certain depth-wise position(hereinafter, this point is referred to as “crossing point”). Thecrossing point depends on the condition of shot-peening, the hardness ofthe steel material, the residual stress distribution of the basematerial of the springs before shot-peening, etc. A tension residualstress due to wire drawing is exerted to the inner surface of ahard-drawn wire produced according to a conventional method.Accordingly, the depth-wise position of the hard-drawn springs aftershot-peening is likely to be shortened compared to a case of springsmade of an oil-tempered wire. On the other hand, it is desirable toapply a strong force at shot-peening to the inventive hard-drawn springsthan springs made of an oil-tempered wire in such a manner that thedepth-wise position of the spring, namely, the crossing point is set asdeep as at 0.05 mm or more, and preferably at 0.15 mm or moreconsidering that hard-drawn springs made of the inventive wire arerequired to be used under a higher stress than hard-drawn springs ofgeneral use.

[0048] In order to set the depth-wise position of the spring, namely,the crossing point at the aforementioned level, it is effective (a) toset the reduction of area of a die used in a final stage of drawing at10% or less, preferably in the range from about 3 to about 6%, (b) toset a stress relieving annealing temperature after coiling at 360° C. orhigher, (c) to perform shot-peening at least once with use of a shothaving an average diameter of 0.3 mm or larger, preferably 0.6 mm orlarger for the purpose of reducing a tension residual stress during awire drawing process.

[0049] In the case where it is expected that the inventive springs beused under a particularly severe and stressful condition, it iseffective to apply a nitriding process to the spring surface. Applying anitriding process makes it possible to further improve fatigue strengthof the springs. Such a nitriding process has been conventionally appliedto valve springs made of an oil-tempered wire. However, there has notbeen applied a nitriding process to conventional hard-drawn springs.This is because heretofore hard-drawn springs have not been used undersuch a severe and stressful condition as required in the recenttechnology, and it has been conceived that a desirable effect cannot beobtained from the conventional hard-drawn wires having a conventionalchemical composition even if nitriding process is applied on resultantsprings.

[0050] Applying a hard drawing process to a wire rod having the chemicalcomposition as defined in this invention and applying a nitridingprocess to the hard-drawn springs enables to improve fatigue life of thesprings. The reason why the hard-drawn springs having been applied witha nitriding process exhibits the above effects is conceived as follows.Specifically, since the inventive springs have low carbon content, thevolume of ferrite phase to cementite phase constituting pearlite isincreased. Furthermore, the strength of the inventive wire depends onthe strength of ferrite itself because ferrite has been strengthened byalloy elements such as silicon, vanadium, and chromium. Accordingly, itis conceived that increasing the strength of ferrite by nitriding leadsto direct improvement of fatigue strength of the springs. The inventorsof this application verified that the effect resulting from performing anitriding process is most remarkably obtainable when the nitridingprocess is performed in such a manner that the depth-wise position at 10μm from the surface of the spring has a hardness of HV600 or more(preferably HV700 or more).

[0051] It is effective to apply a stress τ to springs at least once at aroom temperature or higher, preferably at 120° C. or higher aftershot-peening, wherein the stress τ satisfies the above mathematicalexpression (3) when forming the inventive wire rod or the inventive wireinto the springs. Generally, hard-drawn springs have a low sagresistance compared to springs made of an oil-tempered wire. Thisinvention has been made for the purpose of improving sag resistance ofthe hard-drawn springs by setting the chemical compositions of the steelin the respective predetermined ranges and by increasing the tensilestrength of the wire. However, there is a case that a further improvedsag resistance is required depending on the purpose and condition of useof the hard-drawn springs. To cope with such a demand, it is effectiveto apply the stress τ to springs at least once at a room temperature ormore (preferably, 120° C. or more). It is conceived that applying thestress as mentioned above enables to stabilize dislocation accompaniedby wire drawing and to enhance resistance of the wire against plasticdeformation. It should be noted that the tensile strength TS of thehard-drawn wire in the expression (3) is a value measured with respectto the wire.

EXAMPLES

[0052] Hereinafter, this invention is described in more details withreference to the examples. It should be appreciated that this inventionis not limited to the examples, and as far as not departing from thegist of this invention, any modification and alteration is embraced inthe technical scope of this invention.

[0053] Steel materials (A to I) having the chemical compositionsrespectively as shown in Table 1 were melted, poured into a mold, andsubjected to hot rolling, and steel wire rods each having a diameter of9.0 mm were produced. At this stage, the size of the compoundsprecipitated in the ferrite among the pearlite of each wire rod wasmeasured. The size of the compounds was measured by photographing thecompounds precipitated on the wire rod by thin-film replica method witha transmission electron microscope (TEM) at an accelerated voltage of200KV and magnification of 150,000. Among the precipitating compounds,counted was the number of micro precipitants each having a diameter of50 nm or less in terms of a hypothetical circle which have beenprecipitated in the ferrite of 1 μm² [(150 mm²) at the magnification of150,000]. The site for the measurement was set at the depth-wise 0.2mm-position from the surface of the wire rod in view of the facts that(a) the surface part of a spring is a part where the spring is exertedwith a maximal stress and that (b) the surface of a roll steel is slicedoff by SV process after rolling. Further, the number of complexcarbonitrides, etc. was counted through visual recognition by the TEM.The visually non-recognizable micro complex carbonitrides, etc. havebeen identified as such complex carbonitrides, etc. by means of an X-raydiffraction pattern. The number of complex carbonitirdes, etc. of a sizein a range from 10 to 50 nm was counted by the TEM at a magnification of150,000. Furthermore, the measurement was performed with respect to eachsteel wire rod through arbitrary three different fields of view, and theaverage of the measurement results was obtained (see Table 2).

[0054] After the hot rolling, softening was performed with respect toall the steel wire rods except the steel wire rod made of the steelmaterial F. Then, shaving, patenting, and wire drawing were performedwith respect to all the steel wire rods. Thus, wires each having thediameter as shown in Table 2 were produced. Patenting was performed bysetting an austenitizing heating temperature at 940° C. and by settingthe drawing rate at a relatively high level of 8.0 m/min. Further, withrespect to Examples No. 1 to 10, 12, and 15, the wire rods weresubjected to rapid cooling by forcibly blowing pressurized air onto thewire rods before being carried into a lead furnace at a temperature of620° C. in order to increase the area ratio of pearlite. All the wirerods of the Examples Nos. 1 to 9, 11, and 12 except the wire rod ofExample No. 10 were subjected to isothermal treatment in the leadfurnace, and then cooled at a cooling rate ranging from 0.5 to 1° C./secuntil the temperature of the wire rods has lowered to 400° C. The wirerod of Example No. 10 was cooled at a cooling rate of 3° C./sec untilthe wire rod was cooled to 400° C. after brought into an isothermaltreatment in the lead furnace.

[0055] The wire drawing was carried out by a continuous wire drawingmachine equipped with 8 pieces of dies in which the reduction of area ofeach die except the die used in a final stage of wire drawing was set inthe range from 15 to 25%, and the reduction of area of the die of thefinal use was set at 5%, and the wire drawing rate at the die of thefinal use was set at 200 m/min. Furthermore, cooling wire drawing wascarried out in which the wire rod was directly water-cooled whiledrawing in order to prevent temperature rise of the wire rod accompaniedby wire drawing.

[0056] The thus obtained wires produced by wire drawing were formed intosprings at a room temperature, and subjected to stress relieving (400°C.×20 min.), seat position grinding dual shot-peening, low-temperatureannealing (230° C.×20 min.), and presetting (application with a stress)( τ_(max) corresponding to 1200 MPa). Presetting was performed at about180° C. with respect to the springs of Examples Nos. 4 through 9 byutilizing redundant heat generated from the low-temperature annealing.Nitriding process was applied to the springs of Examples Nos. 5 and 6 at460° C. for 5 consecutive hours. The pearlite area ratio of the springswas analyzed and evaluated by taking photographs of cross sections ofthe wires after patenting by an optical microscope (400 magnifications,10 fields) and by analyzing the photos according to a computerized imageanalyzer. TABLE 1 Kind of Chemical Composition (mass %) Ex. No. Steel CSi Mn Ni Cr V 55 × [C] + 61 Rp[%] 1 A 0.59 1.95 0.88 0.23 0.90 0.11 93.596.5 2 A 0.59 1.95 0.88 0.23 0.90 0.11 93.5 96.5 3 A 0.59 1.95 0.88 0.230.90 0.11 93.5 96.5 4 A 0.59 1.95 0.88 0.23 0.90 0.11 93.5 96.5 5 A 0.591.95 0.88 0.23 0.90 0.11 93.5 96.5 6 A 0.59 1.95 0.88 0.23 0.90 0.1193.5 96.5 7 B 0.51 1.80 0.75 0.08 1.09 0.17 89.1 97.7 8 C 0.65 1.91 0.900.19 0.64 0.09 96.8 98.7 9 D 0.68 1.45 0.75 0 0.40 0.25 98.4 99.0 10  E0.55 1.89 0.81 0.20 0.15 0.08 91.3 98.7 11  J 0.60 1.97 0.79 0.20 1.750.12 94.0 97.1 12  K 0.62 1.85 0.71 0.15 0.70 0.34 95.1 99.0 13  A 0.591.95 0.88 0.23 0.90 0.11 93.5 88.6 14  F 0.92 0.25 0.75 0 0 0 — 100.015  G 0.80 1.90 0.85 0.18 0.85 0.15 — 100.0 16  H 0.80 1.26 0.92 0.320.87 0.20 — 100.0 17  I 0.62 0.96 0.79 0.21 0.96 0.13 95.1 97.9

[0057] Fatigue test was performed with respect to the thus obtainedsprings under a load stress of 637±588 MPa, and the breaking life wasmeasured life was measured with respect to the springs. Further,residual shear strain was measured after fastening the springs under astress of 882 MPa at 120° C. for 48 consecutive hours, and the thusmeasured residual shear strain was set as an index for sag resistance(namely, the smaller the residual shear strain was, the better the sagresistance was).

[0058] The results of measurements are shown in Table 2 along with therespective conditions for manufacturing the springs, tensile strength TSof wire, values in the right object and left object in the expression(2), crossing point, hardness at 10 μm depth-wise position from thespring surface, and the number of precipitants.

[0059] The hardness at the 10 μm depth-wise position from the springsurface was measured by a so-called “Code method” in which a test piecewas embedded in a resin at a known inclination angle, the Vickershardness (load of 300 g) was measured with respect to the test piecewhose surface was polished, and the thus obtained Vickers hardness wasconverted into a corresponding value in a vertical direction. Theresidual stress was measured according to an X-ray diffraction method.The profiles of residual stress with respect to each spring in thedepth-wise direction were evaluated by removing the surface layers ofthe springs stepwise by chemical polishing and by performing X-raydiffraction measurement. TABLE 1 Distance to Hardness at Number of WireTS of Left Right Crossing Stress at 10 μm⁻ Precipitants of ResidualFatigue diameter wire Object in Object in point 120° C. position 50 nmor less in Shear strain Life No. (mm) (Mpa) (2) (2) (mm) or higherNitriding (HV) dia. (per μm²) (× 10⁻⁴) (× 10⁶) 1 4.4 1800 2229 1829 0.15not applied not applied — 20 8.6 7.7 2 3.5 1960 2250 1850 0.15 notapplied not applied — 19 4.2 7.7 3 3.5 1960 2250 1850 0.23 not appliednot applied — 20 4.1 9.6 4 3.5 1960 2250 1850 0.23 applied not applied —20 2.5 9.6 5 3.5 1960 2250 1850 0.23 applied applied 760 23 2.8 10.1  62.2 2113 2359 1959 0.23 applied applied 805 16 2.2 15.5  7 3.5 1890 22501850 0.24 applied not applied — 38 3.1 7.5 8 3.5 1947 2250 1850 0.22applied not applied — 17 2.5 7.8 9 3.5 1875 2250 1850 0.23 applied notapplied — 25 3.0 10.0  10 3.5 2045 2250 1850 0.20 applied not applied — 5 7.9 8.3 11 3.5 1975 2250 1850 0.18 applied not applied — 45 1.7 13.5 12 3.5 1907 2250 1850 0.21 applied not applied — 52 1.9 11.0  13 3.51881 2250 1850 0.23 not applied not applied — 15 7.2  0.96 14 3.5 19152250 1850 0.21 not applied not applied —  6 6.8 2.1 15 3.5 1981 22501850 0.24 not applied not applied — 26 4.6 1.9 16 3.5 1895 2250 18500.22 not applied not applied — 33 7.8 2.3 17 3.5 1925 2250 1850 0.22 notapplied not applied — 15 10.2  5.0

[0060] The following matters have been elucidated based on the resultsof the above experiments. First, although Example No. 1 satisfies theexpression (1), the tensile strength TS of the hard-drawn wire ofExample No. 1 is lower than the value in the right object of theexpression (2) because the reduction of area of the wire at the time ofdrawing is low. Consequently, the spring of Example No. 1 has adeteriorated sag resistance compared to the springs of the otherExamples Nos. 2 to 12. The fatigue life of the spring of Example No. 1is as long as substantially equivalent to the springs of the otherExamples Nos. 2 to 12.

[0061] Examples Nos. 2 to 9, 11, and 12 all satisfy the expressions (1)and (2), and satisfy the requirement regarding the number ofprecipitants each having a diameter of 50 μm or less. The springs ofExamples Nos. 2 to 9, 11, and 12 show excellent fatigue life and sagresistance. Among these Examples, the spring of Example No. 2 differsfrom the spring of Example No. 3 in the aspect of shot-peening condition(namely, the shot used in the first time of shot-peening of the springof Example No. 2 has a smaller size than that in the first time ofshot-peening of the spring of Example No. 3) despite the fact thatExample No. 3 used the same kind of steel material. As a result of thisdifference, the spring of Example No. 2 has a shorter distance to thecrossing point than the spring of Example No. 3, and accordingly, thesprings of Examples Nos. 2 and 3 have substantially the same level ofsag resistance despite the fact that the fatigue life of the spring ofExample No. 2 is shorter than that of Example No. 3.

[0062] The spring of Example No. 4 was obtained by applying a stress tothe spring at 120° C. or higher, whereas the spring of Example No. 3 wasobtained without applying a stress at 120° C. or higher. Although thespring of Example No. 4 has an improved sag resistance, the fatigue lifethereof is substantially the same as the spring of Example No. 3. Thespring of Example No. 5 was obtained by applying a nitriding process,whereas the spring of Example No. 4 was obtained without applying anitriding process. Although the springs of Examples Nos. 4 and 5 havesubstantially the same sag resistance, the spring of Example No. 5 hasan improved fatigue life compared to the spring of Example No. 4.Example No. 6 is different from Example No. 5 in the degree of drawingand the diameter of wire, although the springs of Examples Nos. 5 and 6are substantially the same in the composition of the steel material andthe kinds of treatment. As a result of these differences, Example No. 6has such a high tensile strength as 2113 MPa, as well as improvedfatigue life. Example No. 7 uses a steel material having a relativelylow carbon content, whereas Example No. 8 uses a steel material having arelatively high carbon content. The springs of both Examples Nos. 7 and8 have excellent sag resistance and fatigue properties. Examples Nos. 11and 12 have a relatively high content in chromium and vanadium, and havean increased number of precipitants having a diameter of 50 μm or less,as well as improved sag resistance and fatigue life.

[0063] Example No. 10 satisfies both the expressions (1) and (2).However, the cooling rate until the temperature of the wire rod reaches400° C. after isothermal transformation at the time of patenting is fastand the amount of precipitants decreases with respect to Example No. 10.As a result, the number of precipitants having a diameter of 50 μm orless per unit area is less than ten with respect to Example No. 10. As aresult, the spring of Example No. 10 has a slightly deteriorated sagresistance compared to the springs of Examples Nos. 2 through 9.However, the fatigue life of the spring of Example No. 10 is as long asthe other Examples Nos. 1 to 9, 11, and 12.

[0064] Compared to the above Examples Nos. 1 to 12, Examples Nos. 13through 17 are comparative examples. None of these comparative examplessatisfy at least one of the requirements defined in this invention. As aresult of the experiments, it was proved that Examples Nos. 13 through17 have a deteriorated characteristic with respect to at least one ofthe properties found in Examples Nos. 1 through 12. The chemicalcomposition of the steel material of Example No. 13 is substantially thesame as Examples Nos. 1 through 6. However, since the wire rod ofExample No. 13 was not subjected to cooling by gas at the time ofpatenting, pro-eutectoid ferrite was generated on the wire rod ofExample No. 13, and the pearlite area ratio of the wire rod of ExampleNo. 13 was lowered than the range defined in this invention. As aresult, the fatigue life of the spring of Example No. 13 is remarkablylower than the spring of Example No. 7 although the wire of Example No.13 has substantially the same level of tensile strength TS as ExampleNo. 7.

[0065] The wire rod of Example No. 14 is made of steel in conformancewith the criteria of JIS G3502-SWRS92B. However, the spring of ExampleNo. 14 has deteriorated sag resistance and fatigue life compared to thesprings of Examples Nos. 1 to 12. It is conceived that the reason forsuch a short fatigue life of the spring of Example No. 14 results fromthe fact that using the above steel having a higher carbon contentresultantly raises defect sensitivity and leads to earlier formation offatigue start point. Further, it is conceived that the reason for thelowered sag resistance of the spring of Example No. 14 is due to lesscontent in silicon, chromium, vanadium, etc.

[0066] The steel material of Example No. 15 contains a large amount ofsilicon, and chromium and vanadium are also contained therein. However,since the steel material having high carbon content is used to producethe spring of Example No. 15, the fatigue life of the spring of ExampleNo. 15 is short although sag resistance thereof is good.

[0067] The spring of Example No. 16 contains less amount of siliconcompared to the spring of Example No. 115 (sic). Accordingly, the springof Example No. 16 has deteriorated sag resistance compared to the springof Example No. 15. The steel material of Example No. 17 has carboncontent in the range defined in this invention. However, the steelmaterial of Example No. 17 contains slightly less amount of silicon.Accordingly, although the fatigue property of the spring of Example No.17 is slightly better than the springs of Examples Nos. 14 to 16, thespring of Example No. 17 does not have a fatigue property as high as thesprings of the inventive examples, and sag resistance thereof isremarkably lowered compared to the springs of the inventive examples.

[0068] Industrial Applicability

[0069] This invention is constructed as mentioned above. According tothis invention, realized are a steel wire rod used for producinghard-drawn springs capable of exhibiting fatigue strength and sagresistance equivalent to or higher than springs using an oil-temperedwire, a wire for such hard-drawn springs, such hard-drawn springs, and auseful method for producing such hard-drawn springs at a low cost.

1. A steel wire rod for hard-drawn springs which contains carbon in arange from 0.5 to less than 0.7 mass %, silicon in a range from 1.4 to2.5 mass %, manganese in a range from 0.5 to 1.5 mass %, chromium in arange from 0.05 to 2.0 mass %, and vanadium in a range from 0.05 to 0.40mass %, and has an area ratio Rp with respect to pearlite whichsatisfies the mathematical expression (1): Rp(area%)≧55×[C]+61   (1)where [C] denotes the content (mass %) of carbon:
 2. The steel wire rodaccording to claim 1, wherein the steel wire rod further contains nickelin a range from 0.05 to 0.5 mass %.
 3. The steel wire rod according toclaim 1, wherein ferrite in lamellar in which the number of carbide andcarbonitride of vanadium and chromium, complex carbide and complexcarbonitride of vanadium and chromium each having a diameter of 50 nm orless in terms of a hypothetical circle, is not smaller than ten per μm².4. A wire for hard-drawn springs which contains carbon in a range from0.5 to less than 0.7 mass %, silicon in a range from 1.4 to 2.5 mass %,manganese in a range from 0.5 to 1.5 mass %, chromium in a range from0.05 to 2.0 mass %, and vanadium in a range from 0.05 to 0.40 mass %,and has an area ratio Rp with respect to pearlite which satisfies themathematical expression (1), and has a tensile strength TS of the wirewhich satisfies the mathematical expression (2):Rp(area%)≧55×[C]+61  (1) where [C] denotes the content (mass %) ofcarbon, −13.1d ³+160d ²−671d+3200≧TS≧−13.1d ³+160d ²−671d+2800   (2)where d is a diameter (mm) of the wire which satisfies the expression[1.0 ≦d≦10.0].
 5. The wire according to claim 4, wherein the wirefurther contains nickel in a range from 0.05 to 0.5 mass %.
 6. Ahard-drawn spring producible by using the steel wire rod of claim
 1. 7.A hard-drawn spring producible by using the wire of claim
 4. 8. Thehard-drawn spring according to claim 6, wherein a residual stress of thespring is changed from a compression to a tension at a depth of 0.05 mmor more from an inner surface of the spring.
 9. The hard-drawn springaccording to claim 7, wherein a residual stress of the spring is changedfrom a compression to a tension at a depth of 0.15 (sic) mm or more froman inner surface of the spring.
 10. The hard-drawn spring according toclaim 8, wherein the residual stress of the spring is changed from thecompression to the tension at the depth of 0.15 mm or more from theinner surface of the spring.
 11. The hard-drawn spring according toclaim 9, wherein the residual stress of the spring is changed from thecompression to the tension at the depth of 0.15 mm or more from theinner surface of the spring.
 12. The hard-drawn spring according toclaim 6, wherein the spring is applied with a nitriding process on asurface thereof
 13. The hard-drawn spring according to claim 7, whereinthe spring is applied with a nitriding process on a surface thereof 14.A method for producing the hard-drawn springs of claim 6 comprising thestep of applying a stress τ (MPa) to the spring at a temperature notlower than a room temperature at least once after shot-peening, thestress τ satisfying the mathematical expression (3): τ≧TS(MPa)×0.5   (3)where TS denotes a tensile strength of a wire.
 15. The method accordingto claim 14, wherein the temperature at which the stress τ is applied is120° C. or higher.
 16. A method for producing the hard-drawn springs ofclaim 7 comprising the step of applying a stress τ(MPa) to the spring ata temperature not lower than a room temperature at least once aftershot-peening, the stress τ satisfying the mathematical expression (3):τ≧TS(MPa)×0.5   (3) where TS denotes a tensile strength of the wire. 17.The method according to claim 16, wherein the temperature at which thestress τ is applied is 120° C. or higher.